Tunable high-order harmonic generation and the role of the folded quantum path

We theoretically demonstrate the selective enhancement of high-order harmonic generation (HHG) in two-color laser fields consisting of a single-cycle fundamental wave (800 nm wavelength) and a multicycle subharmonic wave (2400 nm wavelength). By performing time-frequency analyses based on a single-active-electron model, we reveal that such an enhancement is a result of the modified electron trajectories in the two-color field. Furthermore, we show that selectively enhanced HHG gives rise to a bandwidth-controllable extreme ultraviolet supercontinuum in the plateau region, facilitating the generation of intense single isolated attosecond pulses.

Phase-matched high-order harmonic generation in a gas cell with midinfrared femtosecond pulses

We experimentally investigate the generation of high-order harmonics in a 4-mm-long gas cell using midinfrared femtosecond pulses at various wavelengths of 1240 nm, 1500 nm, and 1800 nm. It is observed that the yield and cutoff energy of the generated high-order harmonics critically depend on focal position, gas pressure, and size of the input beam which can be controlled by an aperture placed in front of the focal lens. By optimizing the experimental parameters, we achieve a cutoff energy at similar to 190 eV with the 1500 nm driving pulses, which is the highest for the three wavelengths chosen in our experiment.

Spectral distributions of harmonic generation from electron oscillation driven by intense femtosecond laser pulses

The characteristics of harmonic radiation due to electron oscillation driven by an intense femtosecond laser pulse are analyzed considering a single electron model. An interesting modulated structure of the spectrum is observed and analyzed for different polarization. Higher order harmonic radiations are possible for a sufficiently intense driving laser pulse. We have shown that for a realistic pulsed photon beam, the spectrum of the radiation is red shifted as well as broadened because of changes in the longitudinal velocity of the electrons during the laser pulse. These effects are more pronounced at higher laser intensities giving rise to higher order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that by increasing the laser pulse width broadening of the high harmonic radiations can be limited. (C) 2005 Elsevier B.V. All rights reserved.

Phase-matching control of high-order harmonic generation in a two-color laser field

The phase-matching condition of high-order harmonic generation driven by intense few-cycle pulses could be controlled by adding second-harmonic pulses to change the ionization fraction of the gaseous medium. The harmonic generation efficiency could be improved by moving the phase-matching point with an all-optical control of the ionization fraction or a proper change of the confocal parameter. A specific order of harmonics could be easily controlled to reach phase matching at a fixed higher gas pressure by adding second-harmonic pulses with a suitable intensity. Such an all-optical phase-matching control was demonstrated to be dependent upon the temporal delay between the fundamental-wave and second harmonic pulses.

Measuring ultrashort optical pulses using multi-shot second harmonic generation frequency resolved optical gating

Frequency resolved optical gating (FROG), is an effective technique for characterizing the ultrafast laser pulses. The multi-shot second harmonic generation (SHG) FROG is the most sensitive one in different FROGs. In this paper we use this technique to measure the femtosecond optical pulses generated by a conventional Ti:sapphire oscillator.